Due to rapid progress in the development of organic light-emitting materials, devices based on these materials, called PLEDs and OLEDs (polymer and small-molecule organic light-emitting diodes), are entering the display market. In principle these materials can also be used for large-area lighting applications, which is an important market for the near future. However, the main disadvantages of using PLED/OLED devices for large-area lighting are:
A low-workfunction metal such as Ba or Ca has to be used as cathode to make injection of electrons possible. These metals are very easily oxidized, which shortens the lifetime and requires special packaging of the device. the electroactive layer has to be thin (˜70 nm) because the current, and thus the light output, decreases dramatically with increasing thickness. The processing of large-area layers of such thickness, avoiding shorts and light inhomogeneities, is very difficult.
A very promising alternative to PLED/OLED particularly for lighting applications is the light-emitting electrochemical cell (LEEC) (0). A LEEC does not need a low-workfunction metal electrode and thicker electroactive layers can be used, while keeping the operating voltage low. The operating mechanism is based on the presence of mobile ions.
FIG. 1 schematically shows the operating mechanism of a LEEC; the top pictures are cross sections, and the bottom pictures are energy band diagrams. (a) shows the relative positions of the energy levels when the layers are not in contact: the Fermi levels of the electrodes are not matched with the HOMO and LUMO levels of the electroluminescent layer. The ions in that layer reside in pairs. (b) shows the situation when a voltage is applied high enough to overcome the band gap of the electroluminescent layer: the ions have moved to opposite electrodes so that strong electric field gradients are created, making charge carrier injection and thus electroluminescence possible.
Thus, upon application of a voltage, the cations and anions move towards the cathode and anode respectively, leading to large electric field gradients at the electrode interfaces. The ion distribution formed facilitates injection of electrons and holes at the cathode and the anode respectively, thus allowing transport and recombination of the charge carriers, which results in emission of a photon.
Since the electric field over the electroactive layer is almost completely compensated at the electrode interfaces due to the ion distribution, charge injection is facilitated, even for thick layers. Moreover, matching of the Fermi levels of the electrodes with the energy levels of the electroactive layer is not needed, so that a variety of electrode materials can be used. For instance, non-reactive materials as Au, Ag, Al or ITO can be used as cathode instead of Ba or Ca.
One of the main problems for LEEC is that the performance in terms of efficiency has not yet reached the level of existing light sources or competing technologies for solid-state lighting (i.e. inorganic and organic LEDs). For instance, for a PolyLED containing a green-emitting Ir complex an external quantum efficiency (eqe) of 8% has been achieved (2), whereas for LEECs the eqe is generally in the order of or lower than 1%. Higher efficiencies for LEECs have been obtained but in most cases only at low brightness levels; the efficiency decreases rapidly before a reasonable brightness (e.g. 500 Cd/m2) is reached. As an example, the highest eqe for a LEEC obtained, using a Ru(bpy)32+ derivative, is 5.5% at a brightness in the range of 10-50 Cd/m2 (0). In another case an eqe of 4% at ˜200 Cd/m2 was reported for a polyfluorene, but the device degraded very rapidly (0).
Slinker et al. have described LEECs based on an Ir complex (0). The complex used was [Ir(ppy)2(dtb-bpy)]+(PF6−) (ppy=2-phenylpyridine; (dtb-bpy)=4,4′-di-tert-butyl-2-2′-dipyridyl). Quantum efficiencies of 5% were reported at −3V, i.e. under reverse bias operation. However, the corresponding luminance was only up to 330 cd/m2, which is too low for lighting applications. Higher luminance levels could be obtained by applying a higher voltage (−5V) or by using a low-work function electrode (Ca), but in both cases the efficiency was not high over a reasonable luminance range, and also not stable.
All prior art LEECs thus suffer from the drawback of deteriorated brightness at higher efficiency levels. As a consequence thereof, the performance of LEECs needs to be improved in order to compete with technologies for solid-state lighting.